1. Field of the invention
The present invention relates to a switching power supply circuit.
2. Description of related art
Referring to FIG. 1, there is shown a block diagram illustrating a typical example of a conventional switching power supply circuit. The shown switching power supply circuit is generally designated with Reference Numeral 100, and has an input terminal 110 connected to a primary power supply 200 and an output terminal 120 connected to a load 300. The shown switching power supply circuit 100 includes a controlled switching element 20 connected at its one end to the input terminal 110 so as to receive an electric power from the primary power supply 200, and a smoothing circuit 30 having an input connected to the other end of the controlled switching element 20' and an output connected to the output terminal 120 so as to supply a regulated direct current to the load 300.
The output of the smoothing circuit 30 is also supplied to a voltage conversion circuit 40, where the regulated direct current is converted into a voltage, which is in turn supplied to an inverting input of a comparator 60. A non-inverting input of the comparator 60 is connected to receive a reference voltage from a reference voltage circuit 50. The comparator 60 outputs a voltage signal indicative of which is larger, the regulated direct current voltage or the reference voltage. The voltage signal outputted from the comparator 60 is supplied to an AND circuit (logical product circuit) 80, which also receives a reference oscillation signal from a reference oscillation circuit 70. An output of the AND circuit 8 is supplied to a driver circuit 90 so that the output of the AND circuit 8 is power-amplified. Thus, the driver circuit 90 controls an ON/OFF of the switching element 20 on the basis of the output of the AND circuit 80.
As regards the above mentioned conventional switching power supply circuit, reference can be made to for example "Japanese Translation Version of Motorola: Linear/Switch Mode Voltage Regulator Bankbook", Chapter 13, Jun. 10, 1988.
Now, operation of the circuit shown in FIG. 1 will be described with reference to FIG. 2, which shows a timing chart illustrating voltage change on various points in the circuit shown in FIG. 1.
First, it should be noted that the input of the comparator 60 incorporated in the switching power supply circuit has a voltage hysteresis on the order of several millivolts due to variations in circuit element characteristics and in circuit construction. As is illustrated in the waveform E of FIG. 2, this input hysteresis voltage is extended around the reference voltage V.sub.ref of the reference voltage circuit 50. Here, a high side input hysteresis voltage is expressed as V.sub.ref +V.sub.hys1, where V.sub.hys1 is a difference between the high side input hysteresis voltage and the reference voltage, and a low side input hysteresis voltage is expressed as V.sub.ref -V.sub.hys2, where V.sub.hys2 is a difference between the low side input hysteresis voltage and the reference voltage. Therefore, the hysteresis voltage width or magnitude can be expressed as Vhys.sub.1 +V.sub.hys2.
In this condition, if the output voltage of the voltage conversion circuit 40 is lower than the reference voltage of the reference voltage circuit 50, the comparator 60 continues to output a high level signal (H) until the output voltage of the voltage conversion circuit 40 becomes higher than V.sub.ref +V.sub.hys1, as shown in the waveform B of FIG. 2. During this period, therefore, the output oscillation signal of the reference oscillation circuit 70 as shown in the waveform A of FIG. 2 is supplied to the driver circuit 90 for driving the switching element 20, so that the switching element 20 is forcibly intermittently turned on at the timing defined by the output oscillation signal of the reference oscillation circuit 70, as shown in the waveform C of FIG. 2. At this time, the switching duty becomes a maximum.
When the switching element 20 is driven with the maximum switching duty, the output voltage of the switching voltage supply circuit 100 gradually elevates as shown in the waveform D of FIG. 2. When the output voltage of the voltage conversion circuit 40 becomes higher than V.sub.ref +V.sub.hys1 as shown in the waveform E of FIG. 2, the comparator 60 becomes to output a low level signal (L), with the result that the switching element 20 is turned off. Accordingly, the output voltage of the switching voltage supply circuit 100 gradually drops with time as shown in the waveform D of FIG. 2. The off condition of the switching element 20 is maintained until the output voltage of the voltage conversion circuit 40 becomes lower than V.sub.ref -V.sub.hys2.
When the output voltage of the voltage conversion circuit 40 becomes lower than V.sub.ref -V.sub.hys2 as shown in the waveform E of FIG. 2 as the result of the continuous off condition of the switching element 20, the output of the comparator 60 flips so that the comparator 60 outputs the high level signal, again. As the result of the output change of the comparator 60, the switching element 20 are intermittently turned on with the duty determined by the logical product between the output of the reference oscillation circuit 70 and the output of the comparator 60. Namely, after the output of the comparator 60 becomes the high level signal, again, the switching element 20 is switched to have the maximum duty.
The above mentioned sequential operation is repeated, with the result that the output voltage of the switching voltage supply circuit 100 is maintained substantially at a constant level.
As will seen from the above, however, because of the input voltage hysteresis (on the order of several millivolts) of the comparator used in the switching voltage supply circuit 100, the switching element is, in some case, controlled to have the maximum switching duty during a period of a few cycles of the reference oscillation frequency, and on the other hand to be completely maintained in rite off condition during another period of a few cycles of the reference oscillation frequency.
Under this operating condition, a peak value of the current flowing through the switching element and circuit elements which constitute the smoothing circuit will increase, with the result that the amount of heat generated by those elements correspondingly increases. In addition, a tipple voltage in the output voltage of the switching power supply circuit is larger, as will be apparent from the waveform D of FIG. 2.